As means for solving the problem of an abnormal increase in the amount of the waste plastic materials due to an increased use of plastic materials in recent years, attention has been given to biodegradable plastic materials that undergo the decay by the action of enzymes which are released out of the bodies of bacteria and Eumycetes. Among these biodegradable plastic materials, the polylactic acid is drawing attention as an aliphatic polyester that is easily available being mass-produced on an industrial scale and that is environmentally friendly. Therefore, its use in various forms has been proposed in a wide range of fields.
The polylactic acid (PLA) is a resin made from such starting cereal starches as corns, and is a product obtained by fermenting starches with the lactic acid, or is a polymer obtained by the direct polycondensation of an L-lactic acid as a monomer, or is a polymer obtained by the ring-opening polymerization of a lactide which is a dimer thereof. The polymer is also drawing attention as a resin of the type of a biologically completely recycling system since it can be decomposed into water and carbonic acid gas by the microorganisms present in the natural world.
As a recycling system of the polylactic acid in recent years, the greatest attention has been paid to a chemical recycling method which is capable of decomposing the polylactic acid and reusing it. This method comprises depolymerizing the polylactic acid by the heating in the presence of a depolymerization catalyst, and subjecting the obtained lactide to the ring-opening polymerization again to reuse it as the polylactic acid.
Patent documents 1 and 2 are proposing apparatuses for recovering the lactide from the polylactic acid that is applied to the chemical recycling. According to the apparatuses proposed by these patent documents, the polylactic acid, the depolymerization catalyst and the carrier resin are thrown into a biaxial extruder and are melt-kneaded therein. The melt-kneaded product is then conveyed by a screw in the biaxial extruder into a vent chamber (vent zone) where the lactide formed by the depolymerization of the polylactic acid is gasified, separated from other components and is recovered. Namely, the lactide of a low molecular weight (which is 144) formed by the depolymerization of the polylactic acid has a boiling point of as high as 255° C. under the standard atmospheric pressure. Therefore, upon feeding a molten kneaded product that contains the polylactic acid and the depolymerization catalyst into the vent chamber maintained under a reduced pressure, the boiling point of the lactide can be lowered, and the lactide that is formed can be recovered in a gasified form.
There is no problem if the lactide is recovered by using the above-mentioned recovering apparatuses on a laboratory scale. A problem, however, arouses if it is attempted to recover the lactide on an industrial scale by throwing the polylactic acid in large amounts.
The following facts have been learned through the study conducted by the present inventors. That is, in the extruder, for instance, the carrier resin is moving while being melted and compressed, and the molten polylactic acid having a small melt viscosity and the depolymerization catalyst are conveyed by the carrier resin. Here, when the molten and compressed carrier resin is introduced into the vent chamber in which the pressure has been reduced, the carrier resin and the depolymerized lactide undergo the expansion since the pressure is reduced, and the carrier resin turns into a resin mass and floats on the screw conveyer passage. If the carrier resin grows into a large resin mass, the molten mixture is covered with the resin mass whereby the lactide is prevented from volatilizing. The resin mass, further, clogs the flow passage of the gaseous lactide formed by the depolymerization of the polylactic acid, and causes a great decrease in the efficiency for recovering the lactide. Moreover, the resin mass scatters and mixes into the lactide that is trapped from the vent chamber causing, therefore, serious problems.
The state where the lactide is allowed to volatilize little or is prevented from volatilizing due to the mass of the carrier resin is, usually, called “vent-up”.
The vent up could also be caused by the refluxing of the lactide.
That is, a wall portion (specifically, a cylinder wall forming the screw conveyer passage) in the vent chamber in which the pressure is maintained reduced is heated by a heater, whereby the lactide formed by the depolymerization is gasified and is trapped being separated from the carrier resin and the catalyst. Here, however, the gasified lactide undergoes the condensation upon coming in contact with a peep window (skylight) and with the surfaces of the upper inner wall of a low temperature; i.e., the gasified lactide turns into droplets thereof and often return again onto the screw conveyer passage. If such a refluxing becomes conspicuous, the screws and the surfaces of the cylinder walls are covered with the liquid substance. As a result, the carrier resin (molten resin) undergoes the slipping and no longer moves forward causing the resin mass to grow and the vent-up to occur.
Further, the refluxing phenomenon becomes a process that repeats the gasification and liquefaction, and permits the racemization of the desired lactide to take place. For example, there take place an optical isomeric transition from the L-lactide into the meso-lactide and an optical isomeric transition from the meso-lactide into the D-lactide, causing a decrease in the purity (optical purity) of the obtained L-lactide.